Designing chilled water systems
Typically used for cooling and dehumidifying a building’s air, chilled water (CHW) systems circulate it throughout a building or campus complex. CHW systems also may be used for removing process or other heating loads.
1. Understand the codes and standards that guide CHW system design and energy efficiency requirements.
2. Learn design basics for CHW systems to meet a distribution loop’s load requirements.
3. Understand key equipment and its integration to improve energy efficiency.
Regardless of whether the design is for a new chilled water (CHW) system or a modification to an existing system, an early review of codes, standards, and regulations is necessary to allow for an expedient design and avoid conflicts that will cost time and money to resolve. Local, state, and federal codes and regulations will dictate permitting requirements that affect the location of buildings and equipment (central plants, cooling towers, buried piping systems), fuel handling and storage, environmental emissions and noise, water quality, and safety items.
Groups such as ASHRAE, Air Conditioning, Heating, and Refrigeration Institute (AHRI), the American Society of Mechanical Engineers (ASME), and NFPA all have standards to review for systems, equipment, and testing requirements.
A good primary resource for most engineers today is ASHRAE. ASHRAE’s various technical committees write standards and guidelines to establish consensus for such items as: methods of testing and classification, design, protocol, and ratings for systems and equipment components of those systems. These consensus standards and guidelines are developed by industry leaders with a wide variety of practical and technical/research experience, and published to define minimum values or to encourage acceptable and enhanced performance.
ASHRAE has numerous technical sources of information including a series of four handbooks that are updated every 4 years. Two of these handbooks, Fundamentals – 2013 and HVAC Systems and Equipment – 2012, contain several chapters filled with information and basic criteria needed to design CHW systems. Each handbook has an entire chapter dedicated to listing “Selected Codes and Standards Published by Various Societies and Associations” relevant to the topics covered within the handbooks.
All of the related building system codes—Building Officials Code Administrators International (BOCA) and International Building Code (IBC )—and system components such as piping (ASME B31), ductwork (SMACNA), motors and generators (IEEE, NEMA, UL), and other codes and standards are listed for reference. This is very valuable for any designer or engineer beginning a new project, as these resources are updated every 3 or 4 years.
There are several major components within a CHW system, but chillers are machines filled with refrigerants used in the exchange of heat to “create” and provide the cold water. When chillers are placed in rooms or confined spaces, the designer of the system must incorporate safety provisions to the equipment operator and/or the public. ANSI/ASHRAE Standard 15-2013: Safety Standard for Refrigeration Systems is the reference standard for “machinery rooms” that typically house the larger equipment (i.e., chillers, pumps) necessary for a CHW system. This standard should be used in conjunction with ANSI/ASHRAE Standard 34-2013, Designation and Safety Classification of Refrigerants.
ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency. This standard illustrates minimum efficiency and control systems requirements along with commissioning for building envelope, HVAC, power, lighting, and other equipment, all of which is included in a CHW system design. In ASHRAE 90.1, Chapter 6 is where designers will find minimum energy efficiency requirements for HVAC and CHW system construction with listings for component items such as water- and air-cooled chillers, piping system design flow rates, insulation, and controls.
In addition, ASHRAE also published Guideline 22-2012: Instrumentation for Monitoring Central Chilled-Water Plant Efficiency, which helps designers better understand how to control CHW plants, and has recently developed a District Cooling Guide – 2013 under the auspices of ASHRAE Technical Committee 6.2, District Energy, which does an excellent job of covering items mentioned later in this article.
What is a CHW system?
From the early years of HVAC design, the use of CHW to transfer heat from areas of higher loads (e.g., building loads at air handler coils, or industrial equipment loads at heat exchangers) to a condensing water loop or a refrigeration system for heat rejection has been successful. In a very broad sense, a CHW system consists of the following components:
- A heat absorption component such as a chiller (or evaporator)
- A compressor in a refrigerant cycle
- A heat rejection component such as a cooling tower (or radiator)
- CHW piping
- Either condenser water (CW) piping (for a water-cooled system) or refrigerant based piping (for an air-cooled or evaporative-cooled distribution system) to move the separate fluid systems between the respective components.
Each of the CHW and CW/refrigerant distribution systems will include various additional components and devices such as a pump, a compressor, an expansion tank, air separators/air eliminators, water or refrigerant treatment and filtration devices, isolation and control valves, and a controls system consisting of numerous temperature, pressure, and flow rate metering and control devices. For chillers using air cooling on the condenser side, there is no need for a condenser water loop including piping, cooling tower, and pump. For this article, the fluid systems discussed will be water only.
The CHW portion of the system circulates and flows between the chiller and the building loads through pumping by the CHW pump (although dependent upon the system, usually referred to as the primary pump), and can be operated as constant flow or variable flow. For water-cooled chillers, a condenser water loop is necessary, and always operates when the chiller is energized to operate. This loop also requires a condenser water pump to circulate the CW through the piping between the chiller and the cooling tower or heat rejection device (radiator or closed circuit cooler). The CW system has traditionally been a constant flow (CF) system, but recently designs have included variable flow (VF) in this system as well. Any variable flow application (CHW or CW) increases the intricacy of the design, construction, and operation of a system, but at times of low load and corresponding reduced flow rate requirement, may offer significant pump energy savings. Decisions regarding constant and variable system flows dictate designs typically referred to as primary/secondary (PS) and variable primary (VP) system designs.